Large host landslides commonly encompass smaller, secondary landslides; hence the term “compound landslide.” Secondary landslides differ significantly from their host landslide in that secondary slides (1) have smaller volume; (2) often have greater surface exposure; (3) are more readily saturated by water infiltration; (4) require a smaller driving force to initiate movement; (5) have greater frequency of movement; and (6) their capacity for movement can be either independent of the host and other adjacent secondary landslides, or induced by adjacent landslides. Multilevel flow systems (perched water tables) commonly form within compound landslides due to the relatively low permeability of slip-surface gouge, which may slow recharge from the overlying secondary landslides to the host landslide. Movement-inducing pore-fluid pressures can often be reached more rapidly in secondary landslides. Controlled sources of water, for example, from irrigation and septic waste-water injection systems, can supplement vastly natural recharge from rainfall, artificially maintaining saturation in near-surface secondary landslides.
The progressive movement of a host landslide can generate a family of precursive failure surfaces in the landslide’s interior that can coalesce to form secondary landslides. Evolution of local topography contributes to the morphology and slip direction of secondary landslides to the extent that stress trajectories within a host landslide are perturbed by changes in the shape of the ground surface; for example, increased slope gradient along a landslide’s toe is a common cause of secondary landsliding. The non-plane strain nature of compound landslides, exemplified by the complex interactions between host and secondary landslides, often precludes the meaningful application of general, plane strain, mechanical models in assessing the hazard potential of a specific compound landslide. Rote stability analyses of compound landslides based upon assumed homogeneous, isotropic, linear-elastic behavior throughout the slide prism fail to consider the intrinsically more unstable (and thus more hazardous) secondary landslides.
Geologists involved in landslide exploration must be aware of the characteristics of compound landsliding; those involved in mitigation must consider the hydrologic and dynamic implications of secondary landsliding in order to conduct the requisite field investigations and make appropriate design recommendations, which must be individu-ally suited to each compound landslide.
